Raw element metals are processed into various forms, such as molded shapes, sheet, bar, thin wire or foil, according to applications. In recent years the use of metal powder as molding material is drawing the attention in the fields of powder metallurgy, thermal spraying and other molding techniques. Particularly, powder metallurgy is regarded as an important technology offering wide applications, including production of metal parts, and therefore demand for powder metal—which is the base material for powder metallurgy—is also growing.
Production of metal powder traditionally used the classic method of mechanically and directly crushing metal granules into powder form or the method to blow molten metal with gas pressure to form powder. However, all these and other methods had difficulty achieving uniform granular shape and size, economy, and so on.
Electrolysis is one of relatively new methods for metal powder production. It has been reported that smooth, minute and uniform crystalline structures can be deposited under appropriate conditions, and that performing electrolysis outside the range of these conditions produces brittle metal of sponge or powder form.
Still, these newer production methods did not produce metal particles of satisfactory shape and size uniformity nor did they resolve other problems such as economy.
Among other metals, titanium is a relatively new metal compared with iron, copper and aluminum that have been in use since ancient times. Titanium is light and offers excellent strength at high temperature as well as corrosive resistance, and is therefore used widely in industrial applications.
The sample applications of titanium include jet engine material and structural member for aircraft/spaceship, material for heat-exchangers used in thermal and nuclear power generation, catalyst material used in polymeric chemical products, articles of daily use such as eyeglass frame and golf club head, and material for health equipment, medical equipment and medical/dental material. The applications of titanium are expected to grow further. Titanium, which is already competing with stainless steel, duralumin and other high-performance metals in terms of applications, is likely to surpass its rivals in the future.
Since titanium metal has poor processability and machinability, producing a mechanical part having complex shape from molten titanium will add to manufacturing man-hours and costs. It is because use of molten titanium as material will require cutting and other machining steps following the plastic working process such as hot forging and rolling.
Therefore, powder metallurgy is widely used in titanium metal processing, which is the reason for the growing demand for titanium powder, particularly one offering high purity and good uniformity of granular shape and size. Titanium powder produced by the conventional powder production methods designed for general metals is subject to the same problems with other metals; i.e., irregular granular shape and size, poor economy, and so on. As a result, development of a production method that can provide titanium powder offering high purity and uniform granular shape and size is eagerly awaited.
For example, the hydrogenative dewatering method and rotary electrode method are being put to practical use as improved production methods for titanium metal powder. The hydrogenative dewatering method uses sponge titanium, molten titanium or titanium chips generated from cutting/machining as material. The material titanium is heated in a hydrogen atmosphere to cause it to absorb the hydrogen gas and thus become brittle. This brittle titanium is then crushed and heated again in vacuum so that the hydrogen gas will be released and powder formed. In the rotary electrode method, molten titanium or titanium melted then forged, rolled or otherwise worked is formed into a round bar to be used as material. This material round bar is turned at high speed in an atmosphere of argon, helium or other inert gas, while its tip is melted by a heat source such as an arc or plasma-arc torch. The drips of molten metal are then scattered via centrifugal force to produce spherical powder particles.
The particles of titanium powder obtained by the hydrogenative dewatering method have irregular sphericity. Although this powder can be used in die molding, the heating process must be repeated twice. A crushing process using a ball mill or other mechanical means may be incorporated, but oxygen contamination of titanium powder cannot be avoided. In the rotary electrode method, material titanium is melted in an inert gas and made into powder form. Therefore, particles are spherical and offer good flowability. They are not subject to oxygen contamination, either. However, the solidification property when molded will be reduced. Both methods are a batch system, so the power production cost is high.
The atomization method was developed as a titanium powder production method addressing the aforementioned problems relating to quality and production cost. In the atomization method, material titanium is melted in a water-cooled copper crucible using a plasma-arc torch or other heat source, in order to cause molten titanium to drip continuously from one end of the crucible. Argon, helium or other inert gas is then injected onto the molten titanium to atomize it and obtain powder. However, this method could not reduce the production cost significantly from the levels of the conventional methods, because molten titanium or melted and worked titanium had to be used as material.
In the meantime, a method for producing powder titanium offering improved sphericity and flowability for easier molding, in a manner requiring less cost and avoiding oxygen contamination, is disclosed in Japanese Patent Application Laid-open No. 5-93213. In this method, sponge titanium is isostatically pressed cold into a solid bar. This bar material is then melted in an inert gas, after which argon, helium or other inert gas is injected onto the dripping molten titanium to atomize it and obtain powder. However, this improved method did not offer good purity or uniformity of granular shape and size and the production cost was not at a satisfactory level, either.